Urethane resin compositions formed from a polyol component and a polyisocyanate component have been widely used as a covering material or the like, which is usable for a floor member or the like of an architectural member, since the urethane resin compositions have characteristics such that they are excellent in curability and excellent in elongation of a cured film obtained from the urethane resin composition. However, there are problems in that cured products obtained from the urethane resin compositions are generally soft, and the appearance of a coated film thereof is poor since the coated film is easily foamed due to moisture absorption thereof. Accordingly, in recent years, various studies for improving a hard type urethane resin composition have been conducted (for example, Patent Document 1).
On the other hand, in order to improve the hardness of a covering material, increasing the reactivity of a polyol component and a polyisocyanate component has been accomplished in general. However, when the reactivity is too high, there are problems in that sufficient pot life is not obtained, that is, a sufficient period, wherein a prepared resin composition is maintained in useable condition without being cured, is not obtained, since said components react to each other speedily once both components are mixed.
Therefore, it has been known conventionally that a hard type urethane resin composition, which can be used for a covering, ensures a sufficient pot life, and generates minimal foam even under conditions of high temperature and high humidity, can be prepared by using a mixture, in which of a castor oil fatty acid and a polyol having a structure wherein a higher fatty acid is reacted with a bisphenol type epoxy resin are mixed, as a polyol component, and a mixture, in which diphenylmethane diisocyanate (abbreviated to MDI) and polymethylene polyphenyl polyisocyanate (abbreviated to polymeric MDI) are mixed in a predetermined ratio, as polyisocyanate components (refer to Patent Document 2 shown below).
The aforementioned technique, wherein a mixture of a castor oil fatty acid and a polyol having the structure in which higher fatty acid is reacted with a bisphenol type epoxy resin is used as a polyol component, and a mixture of MDI and polymeric MDI which are mixed in a predetermined ratio are used as a polyisocyanate component, ensures a long pot life and the formation of a hard coating. However, due to the remarkably high viscosity of the polyol component included in the urethane resin composition, it is hard for the technique to be adopted to coating methods such as brush coating, roller coating and spray coating, which can be conducted without skill and do not generate spots on the finished surface after coating.
Furthermore, a polyurethane resin which uses a diol prepared by copolymerizing an oxetane compound with tetrahydrofuran has been known. However, the resin does not ensure a sufficient pot life nor high hardness (refer to Patent document.)    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Sho 57-92015    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2001-187863    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2004-149771